Dark Matter & Black Holes

[Mr Skeptic]

Why doesn't dark matter collapse into a black hole? There's supposed to be these huge disks of dark matter in galaxies that only interact gravitationally -- so what prevents them from collapsing since gravity is only attractive?

[timo]

Dunno, but the ordinary matter of our galaxy isn't clumped to one giant black hole, either.

[Severian]

A lack of friction.

 

It is friction that damps orbits enough for massive compact objects to form. Since dark matter presumably only has a weak interaction there is no effective frictional force.

[Mr Skeptic]

What about evaporative cooling?

[ydoaPs]

A lack of friction.

 

It is friction that damps orbits enough for massive compact objects to form. Since dark matter presumably only has a weak interaction there is no effective frictional force.

 

So, if there is a two body massive dark matter system, instead of colliding due to gravity, there would be harmonic motion?

[halogirl]

i thought that dark matter is only a theory, like the anti-matter, supposedly what blackholes are made of, if there are particles of dark matter floating around that are too small to be black holes,and assuming they are the opposite of matter wouldn't it make sense that they would repel each other. Rather than being sucked down a black hole, the black hole would push it away quickly, doing the opposite of what it does to regular matter, similar to magnetic charges.

[Riogho]

i thought that dark matter is only a theory

,

 

Yes, it is.

 

 

like the anti-matter,

 

Antimatter has been seen.

 

supposedly what blackholes are made of,

 

No, black holes are merely a lot of mass in one area, with lots of gravity.

 

if there are

particles of dark matter floating around that are too small to be black holes,and assuming they are the opposite of matter wouldn't it make sense that they would repel each other. Rather than being sucked down a black hole, the black hole would push it away quickly, doing the opposite of what it does to regular matter, similar to magnetic charges.

 

Nice idea, except... Antimatter and matter attract each other. Dark matter doesn't make up black holes (at least not exclusively, I guess since it interacts via gravity, it would be in black holes).

 

This leads me to an interesting thought, that hopefulyl someone can answer.

 

If dark matter IS in a black hole, would it undergo Hawkign radiation? And if it did, would it radiate as darkmatter particle/anti-particle?

 

>.<

 

Hmmm

[swansont]

only a theory

 

One should become acquainted with what the word "theory" means in scientific discussions, especially on a science board.

 

"A theory is a logically self-consistent model or framework for describing the behavior of a related set of natural or social phenomena. It originates from or is supported by experimental evidence"

http://en.wikipedia.org/wiki/Theory

 

It does not mean "conjecture"

 

"only a theory" is an oxymoron

[halogirl]

in response to swansont: point taken.

 

to: Riogho

good point, i didn't take antimatter into consideration.

[abateNth]

Why doesn't dark matter collapse into a black hole? There's supposed to be these huge disks of dark matter in galaxies that only interact gravitationally -- so what prevents them from collapsing since gravity is only attractive?

 

You need the implication of negative energy if you're wanting to evovle these theories. It's presence has been determined as either existent, or another explanation is responsible.

There's a repulsive force, minimal comparative to others, but in whole pronounced. Something, somewhere, somehow, pushes when everything else pulls. It's measurable.

 

This attractive forces MAY NOT explicitly violate their manner, to end the cycle of what is called propulsion. Whence a form of matter is determined to exist, not easily is it removed. Dark Matter, and the component Energy, do infact become entangled. And, some believe the Star Nursery as it's called is yielding this observance. Where, if you want to observe a phenomenon as Dark Matter, it often is visible in this galaxy cluster, aforementioned, that has little to no momentum in certain areas. None know why, but areas of space are so greatly distorted, many theorize that there's a force "catching" matter; in all forms. Won't really know 'til we get there, or someone from around the area visits and tells us.

[halogirl]

:confused

Why doesn't dark matter collapse into a black hole? There's supposed to be these huge disks of dark matter in galaxies that only interact gravitationally -- so what prevents them from collapsing since gravity is only attractive?

 

 

 

mabe their are multiple forms of gravity, or whatever makes it up repels itself. perhaps there are two parts to gravity, mabe the first part attracts itself strongly, and the other part, weakly repels itself, keeping a balance. it becomes dense but can't quite get that dense due to the second part. sorry to sound vague. also if it collapsed wouldn't it simply become extremely dense? like a huge solid sphere. and if it did that couldn't it be the beginning of a planet?

 

also feel free to correct me. im a biology geek, not physics.:}

[swansont]

:confused

 

 

 

mabe their are multiple forms of gravity, or whatever makes it up repels itself. perhaps there are two parts to gravity, mabe the first part attracts itself strongly, and the other part, weakly repels itself, keeping a balance. it becomes dense but can't quite get that dense due to the second part. sorry to sound vague. also if it collapsed wouldn't it simply become extremely dense? like a huge solid sphere. and if it did that couldn't it be the beginning of a planet?

 

also feel free to correct me. im a biology geek, not physics.:}

 

You don't need to invoke multiple forms of gravity.

 

Severian gave the main reason — dark matter only interacts gravitationally, making it very difficult to shed energy (it could do so only via gravity waves), which, in turn, limits its ability to collapse.

[halogirl]

You don't need to invoke multiple forms of gravity.

 

Severian gave the main reason — dark matter only interacts gravitationally, making it very difficult to shed energy (it could do so only via gravity waves), which, in turn, limits its ability to collapse.

 

 

if it only reacts gravitationally wouldn't it be continually giving off energy in that form?

[swansont]

if it only reacts gravitationally wouldn't it be continually giving off energy in that form?

 

As far as I know, yes, but gravity is very weak — we haven't ever detected gravity waves because they are so hard to measure. So this severely limits the rate at which the dark matter would be able to shed any energy.

[halogirl]

As far as I know, yes, but gravity is very weak — we haven't ever detected gravity waves because they are so hard to measure. So this severely limits the rate at which the dark matter would be able to shed any energy.

 

unless it simply requires a form of technology we havn't developed yet. gravity has to be pretty powerful, it made the planets, its part of the sun.

mabe everything gives off gravity waves mabe it isn't a form of energy.

[ydoaPs]

unless it simply requires a form of technology we havn't developed yet. gravity has to be pretty powerful, it made the planets, its part of the sun.

mabe everything gives off gravity waves mabe it isn't a form of energy.

 

Gravity is BY FAR the weakest force.

[Riogho]

10^-43

[SH3RL0CK]

Since it does interact with gravity (dark matter has been shown to create gravitational lensing), presumably it would get sucked into black holes as does the matter we know about. If we actually knew what dark matter was, we could probably answer your question better.

[Riogho]

I remember seeing somewhere that Dark matter wasn't in black holes... let me go check that out again.

 

Here ya go:

 

Why doesn't dark matter fall into a black hole?

A previously answered question stated that our galaxy could not be "sucked in" to the Black Hole at it's center because of the great distance between it and the nearest matter. If 95% of the universe is composed of "Dark Matter" and ergo also our Milky Way, then why isn't it feeding the Black Hole thereby increasing the event horizon to "someday" include all of the galaxy?

 

You are right that the black hole at the center of the galaxy is increasing in mass as material falls onto it; this causes the Schwarzchild radius to increase a bit as well. However, dark matter very rarely falls into black holes for the same reasons that we don't see it: it interacts very little with itself and with ordinary matter. This is a problem because particles lose a quantity called angular momentum (which is proportional to the speed at which they circle around the galactic center and to their distance from it) by interacting with other particles. Now, the laws of physics say that in order for particles in an orbit around a black hole to fall into it, they must lose a large fraction of their angular momentum (this is essentially because black holes are very small; in order to fall in, then, the particles must get very, very close to the center of the galaxy, which means that they must go very, very fast if angular momentum is conserved). For ordinary particles this is not a problem: as they get closer to the black hole, they bump and rub against each other, losing rotational energy and angular momentum such that they spiral into the black hole (provided they are close enough to the galactic centre to begin with). But dark matter particles don't "bump and rub", by definition! This means that they can't lose angular momentum so they rarely get close enough to the black holes to be "sucked in". So, regardless of the amount of dark matter in the galaxy, it will not help fuel the black hole at its centre.

 

 

http://curious.astro.cornell.edu/question.php?number=358

[halogirl]

Gravity is BY FAR the weakest force.

 

how in any sense is gravity weak. it keeps us stuck to the earth, and the atmosphere, and everything in balance with the sun. it can not be weak.

[ydoaPs]

how in any sense is gravity weak. it keeps us stuck to the earth, and the atmosphere, and everything in balance with the sun. it can not be weak.

 

Ever use a magnet to pick up a paper clip without touching it?

[iNow]

how in any sense is gravity weak. it keeps us stuck to the earth, and the atmosphere, and everything in balance with the sun. it can not be weak.

 

As YDOAP pointed out, the term "weak" is relative. It is weak enough to be overcome by magnetism and other forces, so it is relative to those that we say it's not as potent (or, weaker).

 

The primary reason that we remain on the surface of Earth and don't just float into space, and why the Earth orbits the sun instead of drifting off into the vacuum is due to the differences in mass involved. Relative to us, the earth is Ginormous, and relative to the earth the sun is HUGE. This is why the gravitational pull of the earth on us, and of the sun on the earth, are so meaningful. Regardless of their size, however, gravitation itself is still no match for the the strong, weak, and electromagnetic forces.

[Janus]

how in any sense is gravity weak. it keeps us stuck to the earth, ... it can not be weak.

 

And it takes the whole 6 x 10^24 kg of the Earth's mass to do so. Whereas, as pointed out in the last post, it only takes a small magnet yo lift up a paper clip against the pull of the entire Earth.

 

This is what is meant by gravity being a weak force. It takes a huge mass to create a gravitational force equal to the electromagnetic force created by a very small charge or magnetic pole.

 

Where gravity holds the advantage is in the fact that it only attractive, whereas the electromagnetic forces can be either attractive or repulsive. On a large scale, the negative and positive charges and the North and South poles tend to cancel each other out, leaving gravity to hold sway.

 

One can say that gravity is both rearmost and foremost. Rearmost because of its relative weakness, but foremost because it has the greatest combined effect over large scales.

[thedarkshade]

Gravitational force is the weakest from all four, and that is by an exponential factor!

[Mr Skeptic]

Gravitational force is the weakest from all four, and that is by an exponential factor!

 

Well, sort of. The electrical force between two electrons at one meter distance is [math]\frac{k e^2}{r^2} = 2.30707706 * 10^{-28} Newtons[/math], whereas the gravitational force is [math]\frac{G m_e^2}{r^2} = 5.53806283 * 10^{-71} Newtons[/math]. For an electron then, the electrical force is [math]4 * 10^{42}[/math] times stronger than the gravitational force. [math]4 * 10^{42}[/math] means 4 million billion billion billion billion times stronger. It's not "exponentially stronger" (both decrease by the square of the distance), but it is 42 orders of magnitude stronger.

 

However, it is just as valid to ask "Why is the unit of mass so small compared to the unit of charge?" as "Why is the gravitational force so much smaller than the electrical force?"